Dendrites, also known as whiskers, are microscopic crystalline metallic filaments that grow outwardly in various directions from solder, copper and other similarly susceptible metals. The dendrites have the appearance of very fine hairs which can bridge adjacent parts or conductors, e.g., on a printed circuit board surface between two traces, or between two solder joints, which in both cases have a voltage potential between them. The dendrite formation caused by these metallic dendritic bridges develops over a period of time and may eventually extend between the printed circuit board traces, particularly if the traces are close to each other and/or are unplated copper, because copper is a particularly susceptible metal well known for having electromigration problems. The dendrites can also form between component solder joints which may or may not be on a printed circuit board. This had proved to be a problem in early telephones where the dendrites between components traversed non-trivial distances.
Dendrites are electrically conductive, and when extending from point to point, form an electrical short circuit or on rare occasions, a parasitic resistance. The dendrite structures are caused by electromigration which in turn is caused by a voltage difference acting, e.g., on surface contaminants such as a residue of an acid based solder flux present on a printed circuit board. The problem is exacerbated by the presence of moisture, by a DC bias voltage of greater than about 2.0 volts between the two affected portions, and/or by close proximity bare copper printed circuit board traces. For example, the bias voltage provides a potential that motivates tin particles of the solder to migrate from the positive potential to the negative potential. The short circuit thus produced eliminates the electromigration field generated by the bias voltage.
In a signal processing system having a microprocessor, such as a television receiver, VCR, DVD player or the like, bias is provided by the normal connection of a matrix keyboard, which, e.g., puts a 5.0 volt DC potential via a pull-up resistor between one contact on a momentary contact switch and ground. The problem is exacerbated by the use of “spark gaps” between contacts on the keyboard and ground. In order to protect the input circuitry of the microprocessor from damage caused by electrostatic discharges, such spark gaps, which are two closely spaced bare copper pads with the solder resist removed between the pads, will “fire” at about 2 KV. This area, without solder resist and with a constant bias voltage applied to the keyboard, in combination with residual flux, makes it a prime location for the forming of dendrites. Dendrite formation is also a problem for any closely spaced traces on a printed circuit board, e.g., solder coated closely spaced leads of an integrated circuit.
Accordingly, it is desirable to reduce dendrite formation. An example of measures taken to reduce dendrite formation in the prior art is shown in U.S. Pat. No. 5,872,512 of Kackman et al.